Fuel injection timing accuracy and repeatability are fundamental to diesel engine emissions, fuel consumption, durability and performance. As used herein, the term "fuel injection timing" refers to a point in the standard diesel engine cycle, measured in terms of crank shaft angle relative to piston top dead center (TDC), when fuel is introduced into the combustion chamber of the cylinder. Such fuel introduction is commonly referred to as "start of injection", or SOI. In accordance with typical operation of a diesel engine, SOI may occur several degrees in advance, or retard, of TDC at the conclusion of the compression stroke.
As used above, the term "fuel injection timing accuracy" refers to the uncertainty in establishing a mean SOI condition, wherein the level of uncertainty determines the extent to which desired engine operating conditions can be produced from standard fuel injection system settings. The term "fuel injection timing repeatability", on the other hand, refers to the uncertainty in maintaining a desired SOI condition, wherein the level of uncertainty in this case determines the extent to which desired engine operating conditions can be maintained while fuel injection system settings are held constant.
Fuel system specific definitions and procedures for estimating SOI are necessary to accommodate physical and operational fuel system differences. For example, a closed nozzle unit injector is typically fitted with a needle lift sensor and the instant of needle opening used as an SOI criterion. Although such an arrangement provides for precise closed nozzle SOI data, no such similar arrangement is applicable in an open nozzle fuel injection system due to the structural nature of an open nozzle fuel injector.
An example of one known open nozzle unit fuel injection system 10 is shown in FIG. 1. Referring to FIG. 1, a portion of an internal combustion engine 12 is shown defining a cylinder 14 therein. A piston 16 is disposed within cylinder 14 and the portion of cylinder 14 above piston 16 defines a combustion chamber 15. Piston 16 is attached to a crank shaft 18 which rotates in the direction shown to displace piston 16 within cylinder 14 between a bottom dead center (BDC) position and a top dead center (TDC) position as is known in the art.
Crankshaft 18 is coupled to a camshaft 22, typically via a gear 20, such that camshaft 22 rotates synchronously with the crankshaft 18 in the direction shown. Camshaft 22 defines a non-concentric cam lobe 24 in contact with a rocker arm 26 which is also in contact with a push rod 28. Push rod 28 is, in turn, in contact with a rocker lever 30. Rocker arm 26, push rod 28 and rocker lever 30 together define a so-called injector train.
An open nozzle fuel injector 32, which may typically be a so-called unit fuel injector, includes an injector body 34 defining a bore 36 therethrough. A first injector plunger 38 is disposed within bore 36 and includes a top plate 40. An injector return spring 42 is disposed between injector body 34 and top plate 40 such that plunger 38 is biased against rocker lever 30. A second injector plunger 44 is disposed within bore 36 below plunger 38, and an adjustable hydraulic link 46 is defined therebetween. Alternatively, plungers 38 and 44 can be combined into a single plunger having no hydraulic link therebetween. Bore 36 terminates at its lower end in an open nozzle 48.
As camshaft 22 rotates, the non-concentric cam lobe 24 actuates rocker arm 26 in the directions shown. The action of rocker arm 26 vertically actuates push rod 28 which causes rocker lever 30 to pivot about pivot point 31. The action of rocker lever 30, in turn, imparts a drive force on plunger 38 which is biased toward rocker lever 30 by spring 42. As the force of rocker lever 30 overcomes the biasing force of spring 42, plunger 38 is forced downwardly within bore 36 of fuel injector 32. As the pressure within the portion of bore 36 below plunger 44 is sufficiently increased by the action of descending plungers 38 and 44, a trapped air-fuel mixture is expelled from open nozzle 48 into the combustion chamber 15 of cylinder 14 when the piston 16 is in the vicinity of TDC at the conclusion of the compression stroke as is known in the art. Typically, fuel injection timing is controlled relative to piston TDC by adjusting the angular relationship of the crank shaft 18 and camshaft 22, and/or by adjusting the height of the hydraulic link 46 if fuel injector 32 includes both plungers 38 and 44.
It is generally known in the art that SOI information in an open nozzle fuel injection system, such as system 10 of FIG. 1, can be obtained by measuring the forces imparted to plunger 38 by rocker lever 30 as a function of the position of crank shaft 18, typically measured in degrees relative to piston 16 TDC. To this end, system 10 typically includes a toothed wheel 50 coupled to cam shaft 22 via gear 52. Alternatively, wheel 50 may be coupled directly to cam shaft 22. In either case, wheel 50 rotates in synchronism with cam shaft 22. In other known arrangements, wheel 50 is coupled, either directly or indirectly, to crank shaft 18 for synchronous rotation therewith. Regardless of the specific structural arrangement, wheel 50 ultimately rotates synchronously with crank shaft 18 so that the speed and/or angle of crank shaft 18 relative to piston TDC can be ascertained.
Wheel 50 typically includes a plurality of equally spaced apart teeth 54 and an extra tooth 56 positioned between two of the equally spaced apart teeth 54. A pickup 58 is positioned adjacent wheel 50 to detect the passage of any of teeth 54 and 56 thereby. Tooth 56 is included to provide a means for determining piston TDC, and teeth 54 are used to measure the angle of crank shaft 18 relative to piston TDC. Toothed wheel 50 and pickup 58 define a known engine speed and position sensor which is operable to provide an engine speed/position signal indicative of crank shaft angle relative to piston TDC to computer 60 via signal path 62 connected between pickup 58 and an input port of computer 60.
System 10 further includes a strain gauge sensor 64 attached to rocker lever 30 and connected to an input port of computer 60 via signal paths 66 and 68. Strain gauge sensor 64 is operable to provide an injector train load signal indicative of the load forces imparted to plunger 38 of fuel injector 32 by rocker lever 30 as is known in the art.
Computer 60 simultaneously receives the engine speed/position signal, via signal path 62, and the injector train load signal, via signal paths 66 and 68, and processes these signals as is known in the art to relate crank shaft angle to injector train load as a function thereof. Computer 60 typically further includes additional I/O lines 70 for receiving and sending data relating to the operation of other components of system 10 and of engine operating conditions. Finally, an output device 72, which is typically a plotter, is connected to computer 60 via output lines 74 so that data relating to system 10 can be plotted and thereafter viewed.
Referring now to FIG. 2, a plot of injector train load versus crank angle 80 is shown illustrating a typical open nozzle fuel injection event. The characteristic injector train load curve 80 consists of three distinct phases: (1) train compression 82, (2) transition 84, and (3) homogeneous liquid fuel injection 86. During train compression 82, injector train load increases with downward movement of plunger 38 as spring 42 and other elastic injector train components are compressed and the injection charge, consisting of air, fuel and fuel vapor, is pressurized. Transition 84 follows thereafter during which the air and fuel vapor volumes are collapsed and piston TDC 88 occurs at TDC crank angle 85. It is during transition 84 that SOI occurs at an SOI angle referenced to TDC crank angle 85. The fuel injection event concludes with homogeneous liquid fuel injection 86 during which injector train loads rise sharply and the remaining fuel is expelled from open nozzle fuel injector 32.
A number of subjective criteria for determining SOI information in an open nozzle fuel injection system, such as system 10 of FIG. 1, are known. An example of one such criterion is a so-called Rocker Load Threshold (RLT) approach. The RLT approach defines SOI as the crank angle, measured in degrees relative to piston TDC, corresponding to the point on the injector train load curve that injector train load first achieves a specified threshold level. A graphical example of the RLT approach is shown in FIG. 3.
Referring to FIG. 3, injector train load versus crank shaft angle 80 is shown. The point 88 on the injector train load curve 80 corresponding to piston TDC is shown as occurring within a range 90 of injector train load threshold values. Similarly, the crank shaft angle 85 corresponding to piston TDC is shown as occurring within a range 92 of possible crank shaft angles, wherein the range of possible crank shaft angles corresponds to the range of injector train load threshold values. In accordance with the RLT technique, the SOI crank angle is defined as the crank angle, within crank angle range 92, that corresponds to a predefined injector train load threshold value that occurs within injector train load threshold range 90.
The RLT approach illustrated in FIG. 3 has several drawbacks associated therewith. First, small anomalies in the shallow portion of the injector train load response can produce false SOI indications. While increasing the injector train load threshold value effectively reduces the sensitivity to such anomalies, locating the threshold value above the transition region has the disadvantage that the load and load rate differences between operating conditions produce inconsistencies in estimates of absolute SOI. Secondly, SOI variability is sensitive to the slope of the injector train load response 80 in the transition region and to vertical displacements of the threshold value and load response. Third, the RLT technique requires, as a consequence of inherent subjectivities associated therewith, that an injector train load threshold value to be chosen for a particular operating condition and subsequently applied to all operating cylinders and injection events during the observation period. Compromise is therefore required when SOI variability is great. Further, SOI determination is sensitive to the DC component of strain gauge output for between engine and cylinder comparisons.
Another known subjective criterion for determining SOI information in an open nozzle fuel injection system is a so-called Rocker Load Intersection (RLI) approach. The RLI approach defines SOI as the crank angle, measured in degrees relative to piston TDC, corresponding to the point on the injector train load curve at which best fit lines approximating the slopes of the injector train compression and homogeneous liquid fuel injection portions of the injector train load curve intersects. A graphical example of the RLI approach is shown in FIG. 4.
Referring to FIG. 4, the injector train load response 80 versus crank angle is shown. A best fit line 94 is drawn through the injector train compression portion of response 80 and a best fit line 6 is drawn through the homogeneous liquid fuel injection portion. As shown in FIG. 4, best fit lines 94 and 96 intersect at intersection point 98. In accordance with the RLI approach, the crank angle 100 corresponding to intersection point 98 is the SOI crank angle.
As with the RLT approach, the RLI approach suffers from several drawbacks. First, the RLI approach is largely a manual graphical technique that is often difficult to apply in practice, particularly for operating modes having long transition phases and short homogeneous liquid fuel injection phases. Secondly, the RLI approach ignores the transition phase of the injector train load response, which is commonly held as the phase in which SOI occurs. Rather, the RLI approach depends entirely on the slopes of the train compression and homogeneous liquid fuel injection portions of the injector train load response, which can lead to inherent inaccuracies and variability in SOI determinations.
From the foregoing explanation, it should be apparent that both the RLT and RLI approaches can lead to inaccurate and highly variable SOI determinations. The inherent subjectivity in the selection of the injector train load threshold value in the RLT approach, and in the fit of the straight line segments in the RLI approach, introduce further uncertainty in SOI determinations.
A reference standard is the foundation of any useful measurement approach since it provides a basis for quantitative data comparison. Such a standard, including appropriate definitions and procedures, is necessary if comparisons of SOI are to be made within cylinders, between cylinders, and between engines for the purpose of assessing operational variability. An ideal reference standard should minimize procedural and measurement system contributions to the observed variability and maximize the signal to noise ratio consistent with good measurement practice. What is therefore needed is an objective technique for determining SOI in an open nozzle fuel injection system that minimizes inaccuracies and measurement variability attributable to the technique and maximizes measurement repeatability.